1. Field of the Invention
The present invention relates to a linear channel select filter selecting an IF signal in a satellite broadcasting tuner, and more particularly, to a linear channel select filter controlling a current flowing through a filter to select a desired frequency and having a slim structure reduced in area and size.
2. Description of the Related Art
Generally, a channel select filter is used in a front end part of a receiver to select a channel. An analog filter using one of a switched capacitor filter (SCF), a MOS transistor filter, and an operational transconductor amplifier (OTA), may be used as the channel select filter. In order to use a filter characteristic of a frequency band of tens MHz, the OTA filter may be used as the analog filter. In the OTA filter, a GM-C filter using a transcondutance (GM) is widely used.
In the GM-C filter, while it is a disadvantageous in that a structure of the filter is complicate since it is difficult to control a current while its structure is used in the frequency band of tens MHz. Moreover, it is difficult to realize a method of frequency conversion by one filter as a filtering structure being dependent from a linear characteristic.
FIG. 1 is a block diagram of a conventional variable filter. As shown in FIG. 1, a plurality of filters having the same structure are connected parallel to each other to an input terminal to select one of different frequency channels by changing a filtering frequency in the conventional variable filter or the variable filter need to be redesigned.
For example, when three different frequency bands are required according to application ranges of the variable filter, such as 8 MHz, 18 MHz, and 27 MHz, the variable filter needs three switches SW1, SW2, SW3 and three different filters F1, F2, F3. However, it is a disadvantage that a layout area of the filter on a semiconductor chip increases and accordingly, a manufacturing cost of the semiconductor chip increases.
FIG. 2 is a diagram of a conventional low pass filter (LPF) 130. Referring to FIG. 2, the conventional variable LPF uses an active ladder structure which is widely known, and this active ladder filter is formed based on an RC equivalent circuit corresponding to a passive LC ladder. As shown in FIG. 2, the LPF 130 includes amplifiers A1, A2, A3, A4, and A5, resistors, and capacitors. Since resistances of the resistors are constant, a filter programming ability of the LPF is achieved by using variable capacitors, particularly, by a programmable capacitor array having capacitors C1, C2, C3, C4, C5, C213, C231, C435, and C453. A different type of LPF can be realized to have an operational parameter in a different rage.
FIG. 3 is a circuit diagram of a programmable capacitor array of the LPF shown in FIG. 2. A programmable capacitor array C1 includes twelve parallel capacitors which are user-selectable. FIG. 3 shows a couple of examples of the user-selectable capacitors 300A, 310A, 320A, and 330A which are connected to respective user selectable switches 300B, 310B, 326B, and 330B. A user can select a capacitance of the user selectable capacitors using the respective selectable switches. As shown in FIGS. 2 and 3, a filtered frequency can vary according to the selected capacitance of the user selectable capacitors in the conventional LPF 130.
However, it is impossible to accurately select a desired capacitance using the user selectable capacitors of the programmable capacitor array taking into consideration that the number of the user selectable capacitors cannot be increased indefinitely, and that the number of the user selectable capacitors is limited. Moreover, since a selection of the capacitance is performed non-linearly, it is difficult to accurately select a precise capacitance. Due to these problems, there is a limitation to select the desired of the conventional variable filter.
Since the conventional variable LPF can not be used as a filter requiring a linear frequency selecting performance, a linear filter having an excellent linear characteristic is needed to satisfy a required size and desirable manufacturing cost.